The present invention relates to title and is particularly concerned with method and apparatus for interference cancellation in a high-speed modem.
In a modem designed to receive high-speed data over a physical facility such as a copper pair, ingress of radio signals is a problem, particularly when these radio signals are within or close to the spectrum occupied by the data signal being received by the modem.
A radio signal induces a longitudinal (common-mode) voltage and current on the facility. In the case of a balanced metallic pair, the legitimate signal appears as a differential voltage and current. Ideally, the receiver would not respond to the common-mode signal, and no interference would result. In practice, small asymmetries in the balanced pair cause some transfer of common-mode signal to differential mode. The modem itself may also introduce such imbalance, and in addition the receiver may not entirely reject common-mode signals. The result is that the receiver sees an interfering signal voltage which may be of the order of 10 percent of the common-mode voltage.
An analogous situation occurs in coaxial cable, in which an induced sheath current creates a signal that appears between the center conductor and the sheath.
All of the following techniques are effective, but have certain limitations. In general, Automatic Interference Cancellation, including the version which is the subject of this invention, would be used in combination with these existing techniques:
i) It is possible to reduce the effect of physical asymmetry by using a common-mode choke at the receiver input.
ii) Since a radio signal usually occupies a narrow spectrum compared to the data signal, it is possible to use narrow-band digital and/or analog notch filters to reject the interfering signal Such a filter, if implemented digitally, will introduce a finite but acceptable degradation of the received signal. However, it is ineffective against interfering signals of sufficient amplitude to overload the receiver""s A-D converter or other circuits. An analog filter can prevent such overload, but typically degrades the signal more unless a complex and expensive filter is used. It is also more difficult to make an analog filter adaptive in order to deal with signals appearing at unpredictable frequencies.
iii) A modem using discrete multi-tone modulation can deal with interfering signals of moderate amplitude by simply not using the affected channels. However, it is still vulnerable to overloading by strong interfering signals.
An object of the present invention is to provide an improved method and apparatus for interference cancellation in a high-speed modem.
The present invention takes advantage of the fact that the interfering signal as it appears in the receiver is always a replica of the common-mode signal appearing on the facility, but differs from it in magnitude and phase. Therefore, if we take the common-mode signal and apply the appropriate attenuation and phase shift, we can create a signal equal in amplitude and opposite in phase to the signal as it appears in the receiver, and use this to cancel the interfering signal. This operation is simplified if the interfering signal is narrow-band, since the required attenuation and phase shift will the be nearly the same for all components of the interfering signal.
In accordance with an aspect of the present invention there is provided a method of interference cancellation in a high-speed modem comprising the steps of, responsive to a transmitted signal receiving a differential signal including a received data signal and deriving a common mode signal, deriving orthogonal signals form the common mode signal, deriving orthogonal correction signals in dependence upon the received data signal, orthogonal signals and the common mode signal, multiplying respective orthogonal signals to the differential signal to obtain the received data signal by cancelling interference therein.
In accordance with another aspect of the present invention there is provided apparatus for interference cancellation in a high-speed modem comprising a first coupler for receiving a differential signal including a received data signal in response to a transmitted signal; a second coupler for deriving a common mode signal from the transmitted signal; a phase-shift circuit coupled to the second coupler for deriving orthogonal signals from the common mode signal; first and second multipliers for mixing the orthogonal signals with the received signal; first, second, and third integrators for integrating the mixed orthogonal signals and the common mode signal to produce orthogonal signal level measurement and a common mode signal level measurement, respectively; a controller for deriving orthogonal correction signals from measurements; multipliers for applying the correction signals to the orthogonal signals; and an adder for combining the orthogonal correction signals with the differential signal for cancelling interference therein and deriving the received data signal.
Advantages of the present invention include rapid response to interfering signals such as amateur radio signals.
The present invention will be further understood from the following detailed description, with reference to the drawings in which:
The FIGURE illustrates, in a block diagram, an interference cancellation circuit in accordance with an embodiment of the present invention.